The invention is directed to a method and electrolyte for anodizing valve metals to high voltage.
The dawn of radio broadcasting in the first decades of the twentieth century, and the growing demand for fractional horsepower motors requiring phase-splitting capacitors during this same period, combined to give impetus to the development and sale of high voltage aluminum electrolytic capacitors. These devices, which employ a thin film of anodic aluminum oxide as the capacitor dielectric, have found increasing use throughout the decades of the twentieth century and are now used in electric welders, electric vehicles, and other industrial applications. The relatively high capacitance per unit volume and high voltage capability at relatively low cost also make these devices useful for other applications, such as weaponry (so-called xe2x80x9crail gunsxe2x80x9d, etc.) and medical applications where they are used in implantable defibrillators which are often incorporated into heart pacemaker designs.
The performance and cost effectiveness of aluminum electrolytic capacitors depends upon the production of high quality anodic oxide films on the capacitors anode foils. The production process and electrolytes which have been used to produce these films since the 1930""s are described in the chapters on foil anodizing in the books by Deeley and Georgiev: xe2x80x9cElectrolytic Capacitorsxe2x80x94The Theory, Construction, Characteristics, and Application of All Typesxe2x80x9d by Paul McKnight Deeley, published by The Cornell-Dubilier Electric Corp., South PlainField, N.J., 1938; and xe2x80x9cThe Electrolytic Capacitorxe2x80x9d by Alexander M. Georgiev published by The Technical Division of Murray Hill Books, Inc., New York, 1945.
These works describe the use of a series of electrolyte solutions through which the electrified anode foil is passed to produce the anodic oxide film. The solutions contain boric acid and borax (hydrated sodium tetraborate) in varying concentrations so as to produce a series of electrolyte baths of increasingly higher resistivity. The foil passing through these electrolyte solution baths is biased positive and the baths are biased at progressively higher negative potential such that the anodic oxide film grows progressively thicker as the foil passes through baths containing electrolytes of progressively higher resistivity and biased to progressively higher voltages. The last electrolyte solution may contain only boric acid and have a resistivity of several thousand ohm-cm at 90xc2x0 C. Deeley describes the voltage capability of this method/electrolyte combination as about 800 volts.
The above-cited volumes describe many ionogens which may be employed for anodizing capacitor foil to relatively low voltages (i.e., up to 300 or 400 volts), such as phosphate salts, the salts of organic acids, etc., but high voltage (above about 500 volts) anodizing has been carried-out almost exclusively in aqueous boric acid solutions.
Relatively recently, a method and aqueous electrolyte suitable for anodizing aluminum to approximately 900 to 1000 volts has appeared in the technical literature (xe2x80x9cThe Use of Synthetic Hydrotalcite as a Chloride Ion Getter for a Barrier Aluminum Anodization Processxe2x80x9d, by J. K. G. Panitz, D. J. Sharp, and Brian Melody, Plating and Surface Finishing, December, 1996, pages 52-56). This method basically consists of anodizing aluminum at constant current in a room temperature dilute (high resistivity) solution of ammonium tartrate which has been depleted of chlorine ions down to approximately the 10 ppb level through the use of synthetic hydrotalcite scavenging. While this method of anodizing avoids the use of concentrated boric acid solutions (100 to 200 grams/liter) for the highest voltages, it has proven difficult to apply to reel-to-reel anodizing and is, by its nature, better suited to xe2x80x9cbatch anodizingxe2x80x9d, in which pre-cut foils are anodized while suspended in the anodizing tank (instead of continuously advancing, as in the reel-to-reel anodizing).
It has been realized for many years that high-voltage anodic oxide films of high quality can be readily produced in organic solvent solutions of relatively low free water content. In U.S. Pat. No. 1,710,073, Samuel Ruben describes the use of glycerine solutions of boric acid and borax as a xe2x80x9cfillxe2x80x9d or xe2x80x9cworkingxe2x80x9d electrolyte for electrolytic capacitors. This electrolyte may be used to anodize aluminum to a few hundred volts with good results.
By 1930, Ruben had realized that the inherent limitations of glycerine/borate electrolytes, i.e., the tendency of these electrolytes to oxidize with the production of brown deposits on the anode foil surface during aluminum anodizing above about 150 to 200 volts, and obtained U.S. Pat. No. 1,891,207, which covers the use of ethylene glycol, boric acid, and ammonium borate electrolyte solutions (claim 7 and others). As indicated in the Georgiev book, cited above, (on page 72) properly formulated glycol-borate electrolytes are capable of anodizing aluminum to several hundred volts, with a maximum voltage capability of approximately 600 volts.
The ethylene glycol-based electrolytes must be almost totally esterified to the glycol-borate polyester for use as anodizing electrolytes above about 400 volts. If a significant amount of free ethylene glycol is present, the anodic oxidation products (oxalic acid, formic acid, etc.) of the glycol will attack the aluminum anode material. This corrosive attack by ethylene glycol oxidation products is described in xe2x80x9cThe Potential for Positive Tab Corrosion in High Voltage Aluminum Electrolytic Capacitors Caused By Electrolytic Decomposition Productsxe2x80x9d, by Brian Melody, presented at the 1993 Capacitor and Resistor Technology Symposium, pages 199-205, Symposium Proceedings. 
The anodizing voltage range of organic solvent-based electrolyte solutions was greatly extended by Julius Edgar Lilienfeld, who described the preparation and use of the esterification (condensation) product of boric acid with triethylene glycol in U.S. Pat. No. 2,013,564. Lilienfeld later extended this work to include other glycols, such as diethylene glycol, and other acids in addition to boric acid, such as citric acid and tartaric acid, in U.S. Pat. No. 1,986,779. The highest anodizing voltage capability of these materials is associated with the borate esters of glycols having at least 2 ethylene oxide groups (i.e., diethylene glycol, triethylene glycol, etc.)
The present inventors duplicated the borate polyester formed by the reaction of triethylene glycol with an equivalent amount of boric acid. This polyester forms upon heating the two ingredients to 130 to 160xc2x0 C. to drive off the water produced by esterification. It was found that triethylene glycol borate polyester solution may be used to anodize aluminum foil to 1,500 volts, at a temperature of 150xc2x0 C. to 160xc2x0 C. (1 mA/cm2 current density). This oxide thickness is equivalent to over 1,790 volts at 85xc2x0 C. or approximately 1,980 volts at 50xc2x0 C.
Thus, polyglycol borate esters, although well suited for the high voltage anodizing of aluminum for high voltage electrolytic capacitor use, are not suitable as anodizing media. Borate polyesters of ethylene glycol, glycerine, diethylene glycol, triethylene glycol, and the like are all solid at room temperature. These borate polyesters must be heated to temperatures substantially above the boiling point of water to render them sufficiently fluid for anodizing use. For example, the borate polyester of triethylene glycol must be heated to 150 to 160xc2x0 C. for sufficient fluidity for anodizing. Moreover, these materials tend to become increasingly more viscous with time at elevated temperatures, limiting their service life.
It is an object of the invention to produce a borate polyester product that remains liquid at temperatures as low as 25xc2x0 C. or even lower.
It is another object of the invention to produce a high resistivity liquid (in excess of 100,000 ohm-cm) capable of producing a very high voltage anodic oxide on valve metals, particularly aluminum.
It is another object of the invention to provide a borate polyester solution that does not require heating to high temperatures for anodizing.
It is another object of the invention to provide a borate polyester solution that allows anodizing of aluminum substrates at temperatures of about 25 to 85xc2x0 C.
The invention is directed to a method of anodizing valve metals with a borate polyester solution formed by combining 2-methyl-1,3-propane diol and boric acid and heating to about 130 to about 160xc2x0 C. which drives off the water produced by esterification.
The invention is further directed to a borate polyester solution prepared by reacting 2-methyl-1,3-propane diol and boric acid.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention as claimed.
It was discovered that 2-metbyl-1,3-propane diol reacts with boric acid to produce a borate polyester product which remains liquid at temperatures as low as 25xc2x0 C. or even lower. Because of the relatively high melting/softening points of other borate polyesters, it was unexpected that the borate polyester condensation product of boric acid and 2-methyl-1,3-propane diol would remain a liquid at room temperature. While not wishing to be bound by any particular theory, it is believed that the relatively low melting point of the 2-methyl-1,3-propane diol/polyborate is due to the stearic hindrance to hydrogen bonding resulting from the methyl group present in the diol structure.
This electrolyte composition formed by the thermal condensation product between boric acid and 2-methyl-1,3-propane diol is also easier to handle than other glycol-borates due to an inherent resistance to hardening due to loss of the solvent from the solution. Although half or more of the solution volume may be lost to evaporation, the 2-methyl-1,3-propane diol borate will remain fluid at least down to about 25xc2x0 C.
Thus, in one embodiment, the invention is directed to a borate polyester electrolyte solution formed by the reaction of 2-methyl-1,3-propane diol and an equimolar amount of boric acid. The polyester is formed upon heating the 2-methyl-1,3-propane diol and boric acid to 130 to 160xc2x0 C., preferably about 135 to about 145xc2x0 C. Heating to this temperature drives off water produced by esterification. Although the diol and boric acid are reacted in equi-molar amounts, an excess of either ingredient may be present without harming the reaction and any excess amount that does not materially affect the reaction is within the scope of the invention. For instance, if excess diol is present, the excess is evaporated during the heating. Further, excess boric acid will not materially affect the results since the excess tends to form polyborate which ultimately dissolves in the solution. Generally, up to about 10 weight percent excess boric acid can be tolerated.
It was further discovered that the condensation product formed between boric acid and 2-methyl-1,3-propane diol upon heating to about 130xc2x0 C. to 160xc2x0 C. is a high resistivity liquid (in excess of 100,000 ohm-cm) capable of producing a very high voltage anodic oxide on valve metals, particularly aluminum.
In another embodiment, the invention is directed to anodizing a valve metal substrate using the borate polyester electrolyte solution formed between boric acid and 2-methyl-1,3-propane diol.
The low melting point of the boric acid and 2-methyl-1,3-propane diol thermal condensation product and the relatively low viscosity above the melting point allows for a reduction of the anodizing temperature. That is, typical anodizing temperatures required for the condensation products of boric acid with, for example, triethylene glycol (150 to 160xc2x0 C.) may be reduced to about 25 to 85xc2x0 C. by employing the electrolyte of the present invention, realizing a large savings in heat supplied to the anodizing vessel.
The thermal condensation product of boric acid and 2-methyl-1,3-propane diol (i.e., the 2-methyl,1,3-propane diol-borate polyester) is particularly well suited to the growth of high voltage anodic valve metal oxide films, in particular aluminum oxide films. Anodic oxide films may be produced on aluminum at voltages as high as 3,000 volts, or even higher, through the use of this electrolyte. The invention may be employed for low voltage applications as well.
Although not limited to these temperatures, the present method is preferably operated in the temperature range of about 25 to about 85xc2x0 C. The highest voltage anodic oxide films require lower anodizing temperatures of 25-50xc2x0 C., while films formed at higher current densities and to somewhat lower voltages should be produced at temperatures of 50-85xc2x0 C., where the lower viscosity allows a rapid escape of gas bubbles and the lower resistivity gives rise to a more uniform anodic oxide film thickness in a relatively short period of time.
The preferred operating temperature range of the electrolyte/method of the invention represents a large improvement over prior processes/electrolytes capable of operating at high voltages (above about 1,000 volts) because prior art electrolytes based upon boric acid/polyol condensation products required temperatures in excess of about 150xc2x0 C. for sufficient fluidity for anodizing purposes.
The 2-methyl-1,3-propane diol may be obtained from Arco Chemical Company, for example, under the trade name of xe2x80x9cMPDiolxe2x80x9d.
Although any valve metal may be used, the electrolyte and method of the invention are particularly useful for aluminum substrates.